Scuba Tanks & Valves — PADI IDC / DM Study Notes
Equipment — Topics
Steel vs Aluminium Tanks
Steel is stronger than aluminium, so a steel tank can be made with thinner walls to hold the same amount of air. Thinner walls mean a physically smaller tank. And a smaller tank displaces less water — which means less buoyancy. That chain of logic explains every steel vs aluminium buoyancy difference you will see in the exams.
| Property | Steel tank | Aluminium tank (e.g. AL80) |
|---|---|---|
| Wall thickness | Thinner | Thicker |
| Physical size (same capacity) | Smaller | Larger |
| Weight out of water | Lighter to carry | Heavier to carry |
| Buoyancy in water | More negative | More positive |
| Buoyancy when empty | ~2.5 lb negative | ~4 lb positive |
| Lead weight needed | Less | ~6.5 lb more than equivalent steel |
| Corrosion risk | Higher (rust in salt water) | Lower (aluminium oxide — white) |
Servicing — Visual Inspection vs Hydrostatic Test
| Test type | Frequency | What it involves |
|---|---|---|
| Visual inspection | At least annually | Valve removed; inside and outside inspected for corrosion, pitting, cracks, dents, and galvanic action |
| Hydrostatic test | Every 5 years (or per local law) | Tank pressurised to 150% of working pressure; permanent expansion measured |
What inspectors look for in a visual inspection
- Corrosion / oxidisation — rust inside a steel tank; white aluminium oxide inside an aluminium tank
- Pitting — small craters in the metal wall, usually caused by corrosion
- Cracks — most likely found on the inside of the shoulder, around the neck thread; a light and mirror are used to see them
- Dents — usually on the outside from mishandling
- Galvanic action — corrosion where the valve contacts the tank (e.g. a brass valve in an aluminium tank). Happens when two electrochemically dissimilar metals are in contact with a conductive path between them — one metal loses ions to the other and corrodes.
How hydrostatic testing works
The tank is placed inside a water-filled jacket, pressurised to 150% of working pressure, and held there. The pressure causes the tank to expand slightly, pushing water out of the jacket and into a measuring beaker. After pressure is released, the beaker is weighed again to find the permanent expansion — how much the tank grew and did not spring back.
Both the jacket and the tank are filled with water rather than air for a critical safety reason. Water is incompressible — it stores almost no energy under pressure. If a tank filled with air were to fail during the test, the compressed air would explosively expand and destroy the test facility. Fill both with water, and a failure simply cracks the tank. Same test result, no explosion.
Permanent expansion of less than 10% of total expansion = pass. More than 10% = fail.
Other reasons to take a tank for inspection
- Something is rattling inside the tank — most commonly a loose tank snorkel (the small tube inside the valve that prevents contaminants reaching the regulator)
- The tank feels heavier than normal — likely water ingress, which causes corrosion and makes the tank unsafe to fill
Tank Markings
Steel tank markings
| Marking | Meaning |
|---|---|
| 3A | Carbon steel |
| 3AA | Chrome molybdenum steel — the more common steel alloy used today |
| + (plus sign) | A 10% overfill is permitted (e.g. working pressure 2250 psi → can fill to ~2500 psi) |
| DOT / CTC | Government agency approval (US/Canada); confirms the tank meets transport regulations |
Aluminium tank markings
| Marking | Meaning |
|---|---|
| 3AL | Aluminium alloy — the most common marking on aluminium scuba cylinders |
Nitrox markings
- Yellow and green stickers identifying the tank as nitrox
- Visual inspection sticker (month and year punched out)
- Oxygen clean sticker if applicable (if absent, assume not oxygen clean)
- A label showing: gas blend, maximum operating depth, diver's name/signature, date, and fill pressure
Tank Valves
K valve
The standard valve on virtually every modern scuba cylinder. A simple on/off valve with a single regulator port. Nothing complicated to know — it is the default.
J valve
Rarely seen today, but tested in PADI exams. Before submersible pressure gauges existed, divers had no way of knowing how much air remained. The J valve solved this with a simple mechanical warning system:
- A spring-loaded mechanism inside the valve, with a lever on the outside set to the up position at the start of the dive
- When tank pressure drops to roughly 300–700 psi, the spring closes the valve — the diver suddenly cannot breathe
- The diver reaches behind and pushes the lever to the down position, releasing the remaining reserve air
- That restricted breathing is the warning: you are now on reserve — end the dive
Twin tank manifolds (dual valves)
Used primarily in technical and decompression diving, a dual manifold connects two tanks with an isolator valve in the middle. Both regulators can draw from both tanks evenly under normal conditions. In an emergency:
- Regulator failure / free-flow — shut the valve on the failed regulator side. You can now breathe both tanks through the working regulator only.
- Tank O-ring failure (leaking neck) — close the isolator valve to separate the two tanks. Continue breathing the failing tank dry on its own regulator, then switch to the good tank. This preserves as much air as possible from the undamaged side.
Burst Discs
Every tank valve contains a burst disc — a thin copper disc designed to rupture and vent air safely if the tank is overfilled or overheated. It is a deliberate weak point: better the disc fails in a controlled way than the tank fails catastrophically.
- Burst pressure: approximately 140% of working pressure
- The retaining nut has multiple holes so air vents in several directions — preventing the tank from spinning or rocketing when the disc bursts
- Burst discs weaken over time and must be replaced annually as part of routine servicing — an old disc can rupture below working pressure
Equipment — Topics